Child having bruising all over body and red marks around eyes. Having thrombocytopenia. Underlying causes??

my son has noted over the past several weeks he is having more bruising or ecchymosis all over his body after coughing this morning he noticed tiny red marks all around his eyes . my son has thrombocytopenia. what is the blood disorder and what does plasma red blood cells and platelets have to do with this what is the cause of this disorder